In the field of medicine or the like, an X-ray imaging device that emits X-rays and generates an X-ray image is widely used to diagnose inside of a subject. X-ray images which have been generally spread are generated using an absorption imaging method of making a difference in attenuation of X-ray intensity into an image as a contrast.
X-rays emitted to a subject are absorbed and attenuated depending on materials constituting parts of the subject at the time of being transmitted by the subject. X-rays transmitted by a subject are detected as an X-ray absorption image by an X-ray detector and are output as an X-ray detection signal. Since an intensity of an X-ray detection signal varies depending on an X-ray absorption factor, an X-ray image in which a difference in attenuation of an X-ray intensity is expressed as a contrast (a difference in gray level) is generated by performing various image processes on the X-ray detection signal. For example, since a bony tissue has a high X-ray absorption factor, an image of a bony tissue with a high contrast can be acquired using an absorption imaging method.
However, an X-ray absorption factor varies greatly depending on elements constituting a subject and an element having a small atomic number has a small X-ray absorption factor. A soft tissue such as a cartilage including many elements having small atomic numbers hardly absorbs X-rays. Accordingly, it is difficult to acquire an image of a soft tissue with a sufficient contrast from an X-ray image formed using an absorption imaging method.
Therefore, recently, techniques of imaging a subject using a phase difference of X-rays or refraction of X-rays have been proposed (for example, see Patent Documents 1 and 2). X-rays which are a kind of electromagnetic waves have different propagation speeds inside and outside of a subject. Therefore, as illustrated in FIG. 17, phases of X-rays are shifted and waveforms S of the X-rays are changed as indicated by an arrow Q when X-rays are transmitted by a subject M (see reference sign R). As a result, a phenomenon in which traveling directions of X-rays are refracted (scattered) occurs. That is, X-rays P1 which are not transmitted by the subject M propagate straight and X-rays P2 which are transmitted by the subject M are refracted depending on a shape or a constituent material of the subject M or the like.
Actually, a refraction angle of an X-ray is a small angle which is one over several thousands, but an X-ray refraction effect is much larger than an X-ray attenuation effect. Accordingly, an X-ray image with a high contrast can be acquired for a soft tissue having a low X-ray absorption factor or the like by measuring refraction of X-rays due to transmission by a subject. An X-ray image in which a refraction contrast image is mirrored and which is acquired based on refraction information of X-rays due to transmission by a subject is referred to as a small-angle X-ray scattered image. As a technique of capturing such a small-angle X-ray scattered image, an edge illumination X-ray phase contrast imaging (EI-XPCi) method has been recently proposed (for example, see Non-Patent Document 1).
A configuration of a conventional X-ray imaging device that captures a small-angle X-ray scattered image by EI-XPCi will be described below. As illustrated in FIG. 18(a), a conventional X-ray imaging device 101 which is used for EI-XPCi includes an X-ray tube 103 that emits X-rays 103a to a subject M, an X-ray detector 105 that detects the X-rays 103a, a sample mask 107, and a detection mask 109. The sample mask 107 is disposed between the subject M and the X-ray tube 103. The detection mask 109 is disposed at a position close to the X-ray detector 105 between the subject M and the X-ray detector 105.
As illustrated in FIG. 18(b), each of the sample mask 107 and the detection mask 109 has a configuration in which X-ray absorbing materials R1 that extend in a y direction and absorb X-rays and X-ray transmitting materials R2 that extend in the y direction and transmit X-rays are alternately arranged. A pitch T in the sample mask 107 and the detection mask 109 ranges, for example, from about 60 μm to 100 μm and the X-ray absorbing materials R1 and the X-ray transmitting materials R2 have substantially the same length in an x direction.
A flat panel type detector (FPD) or the like is used as the X-ray detector 105. Here, an indirect conversion type X-ray detector that converts X-rays into light using a scintillator element or the like and converts the light into electric charges which are an electrical signal will be described as an example. As illustrated in FIG. 19(a), the X-ray detector 105 has a configuration in which a scintillator layer 105a and an output layer 105b are stacked in a z direction. The scintillator layer 105a includes scintillator elements that absorb X-rays and convert the absorbed X-rays into light.
The output layer 105b includes a substrate 111 and pixels 113 that are arranged in a two-dimensional matrix shape. Each of the pixels 113 includes a photoelectric conversion element and an output element which are not illustrated. In the x direction, the pixels 113 are arranged to correspond to the X-ray transmitting materials R2 of the detection mask 109 in a one-to-one manner.
A part of X-rays emitted from the X-ray tube 103 in the z direction are absorbed by the X-ray absorbing materials R1 of the sample mask 107 and the X-rays are limited to a fan beam shape of which a length in the x direction corresponds to the length of the X-ray transmitting materials R2 and which extends in the y direction. X-rays with a fan beam shape transmitted by the X-ray transmitting materials R2 of the sample mask 107 are incident on the subject M. The X-rays transmitted by the subject M are incident on the detection mask 109 and a part thereof is absorbed by the X-ray absorbing materials R1 disposed in the detection mask 109. X-rays which are transmitted by the X-ray transmitting materials R2 of the detection mask 109 and which are shaped into a fan beam shape narrower in the x direction are incident on the X-ray detector 105.
X-rays incident on the X-ray detector 105 are converted into light in the scintillator layer 105a and are emitted as scintillator light. The scintillator light is transferred to the pixels 111, is subjected to photoelectric conversion by photoelectric conversion elements disposed in the pixels 111, is converted into electric charges which are an electrical signal, and is output as an X-ray detection signal from the output elements. An X-ray image is generated based on the output X-ray detection signal.
When a small-angle X-ray scattered image is captured by EI-XPCi, an X-ray image is captured while moving the sample mask 107 and the detection mask 109 to be relative to each other. That is, X-rays are emitted with a positional relationship illustrated in FIG. 19(b), an X-ray image A1 is captured, and then the detection mask 109 and the X-ray detector 105 are further moved in the x direction. For example, the moving distance corresponds to half the pitch T of the detection mask 109. As illustrated in FIG. 19(c), after the detection mask 109 and the X-ray detector 105 are moved in the x direction, X-rays are emitted again and an X-ray image A2 is captured.
X-ray refraction information based on the subject M can be acquired using the X-ray image A1 and the X-ray image A2 which have been captured while relatively moving two masks. That is, X-rays P1 which are not transmitted by the subject M among X-rays transmitted by the X-ray transmitting materials R2 of the sample mask 107 are not refracted. Accordingly, in the X-ray image A1 and the X-ray image A2, a dose of X-rays P1 incident on the X-ray detector 105 is constant regardless of whether the subject M is present.
On the other hand, X-rays P2 among X-rays transmitted by the X-ray transmitting materials R2 of the sample mask 107 are refracted due to transmission by the subject M. Accordingly, the dose of X-rays P2 incident on the X-ray detector 105 in the X-ray image A1 and the X-ray image A2 increases or decreases depending on a refraction angle of X-rays P2 in comparison with the dose of X-rays P1 incident on the X-ray detector 105. Accordingly, by performing various processes of calculating a difference between both images on the X-ray image A1 and the X-ray image A2, a small-angle X-ray scattered image based on the X-ray refraction information is generated. In this way, by capturing a plurality of X-ray images while relatively moving the sample mask 107 and the detection mask 109, it is possible to acquire a small-angle X-ray scattered image of a subject M.